Abstract

Bacterial utilization of allochthonous organic carbon and the subsequent fate of the bacterial production in the pelagic food web were estimated in Lake Örträsket, a large humic lake in northern Sweden. Bacterial production relied mainly on allochthonous DOC and exceeded primary production in the epilimnion. Bacterial productivity was clearly stimulated during high flow episodes, increasing the input of bacteria degradable organic material to the lake. Bacterioplankton were exploited mainly by mixotrophic flagellates which probably used bacteria as a source of carbon, phosphorus and nitrogen. The possibly extremely low availability of inorganic P during periods with high bacterial production may have allowed the mixotrophs to outcompete obligate autotrophs and help them become dominant phytoplankters during large parts of the summer. The results from Lake Örträsket indicated that the total production depended on bacterial energy mobilization from allochthonous organic carbon compounds and that heterotrophic mobilized energy was linked via mixotrophs to higher levels in the food chain.